U.S. patent number 5,393,592 [Application Number 08/003,939] was granted by the patent office on 1995-02-28 for heat-sealable plastic film.
This patent grant is currently assigned to BASF Aktiengesellschaft. Invention is credited to Helmut Jenne.
United States Patent |
5,393,592 |
Jenne |
February 28, 1995 |
Heat-sealable plastic film
Abstract
A heat-sealable plastic film produced by coextrusion or
lamination, comprising 2 layers A and C and, if desired, a layer B
and, if desired, a layer of an adhesion promoter for bonding each
two of the layers A, B (if present) and C (the sum of the
thicknesses or of the weights of A, B (if present) and C in each
case being 100) comprises from 1 to 50 % of a layer of a
heat-sealable, impact resistant polystyrene A, up to 95% of a base
layer B and from 1 to 99% by weight of a high-melting plastic layer
C.
Inventors: |
Jenne; Helmut (Schriesheim,
DE) |
Assignee: |
BASF Aktiengesellschaft
(Ludwigshafen, DE)
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Family
ID: |
6384101 |
Appl.
No.: |
08/003,939 |
Filed: |
January 19, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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537697 |
Jun 14, 1990 |
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Foreign Application Priority Data
Current U.S.
Class: |
428/213;
428/476.1; 428/483; 428/475.8; 428/349; 428/521; 428/517 |
Current CPC
Class: |
B32B
7/12 (20130101); B32B 27/08 (20130101); B32B
27/302 (20130101); B32B 37/153 (20130101); B32B
2325/00 (20130101); Y10T 428/2826 (20150115); Y10T
428/31917 (20150401); Y10T 428/31797 (20150401); Y10T
428/31931 (20150401); Y10T 428/31746 (20150401); B32B
2307/31 (20130101); Y10T 428/2495 (20150115); Y10T
428/31743 (20150401) |
Current International
Class: |
B32B
27/08 (20060101); B32B 027/08 (); B32B 027/34 ();
B32B 027/36 (); B32B 031/30 () |
Field of
Search: |
;428/213,349,475.8,476.1,483,517,521 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1084919 |
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Jul 1960 |
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DE |
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1420698 |
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Oct 1968 |
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DE |
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1934348 |
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Jan 1970 |
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DE |
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1645406 |
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May 1970 |
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DE |
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1959922 |
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Jul 1971 |
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DE |
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2550227 |
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May 1977 |
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DE |
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3030364 |
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Mar 1981 |
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DE |
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3531036 |
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Mar 1987 |
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DE |
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2029766 |
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Mar 1980 |
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GB |
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Primary Examiner: Nakarani; D. S.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt
Parent Case Text
This application is a continuation of application Ser. No.
07/537,697, filed on Jun. 14, 1990, now abandoned.
Claims
We claim:
1. A heat-sealable plastic film produced by extrusion or
lamination, comprising:
a layer A of a heat-sealable mixture of three components A.sub.1,
A.sub.2 and A.sub.3, wherein
component A.sub.1 is an impact resistant polystyrene consisting of
a styrene polymer resin matrix and a phase which is a graft
copolymer of styrene and a rubber selected from the group
consisting of polybutadiene; an elastomeric linear
styrene-butadiene two-block copolymer and mixtures thereof,
component A.sub.2 is a styrene-butadiene or styrene-isoprene block
copolymer, and
component A.sub.3 is a lubricant;
a layer B of an impact resistant polystyrene resin consisting of
from 60 to 95% by weight of a polymer selected from the group
consisting of polystyrene, homopolymer of substituted styrene,
copolymer of styrene and maleic anhydride, and copolymer of styrene
and methylmethacrylate and from 5 to 40% by weight of an
elastomeric particulate phase of polybutadiene, styrene grafted
onto polybutadiene, styrene-butadiene two-block copolymer or
styrene grafted onto linear styrene-butadiene two-block copolymer;
and
a layer C of a high-melting plastic film selected from the group
consisting of polycaprolactam, polyhexamethylene adipamide,
copolyamide of caprolactam, hexamethylenediamine and terephthalic
acid and polybutylene terephthalate.
2. The plastic film of claim 1, which further comprises a layer D
consisting of either a copolymer of styrene and maleic anhydride or
a block copolymer of styrene and butadiene which is located between
layers A and B, between layer B and layer C, or between layers A
and B and C.
3. The plastic film of claim 1, wherein said film is constituted of
from 3-30% of said layer of Component A, from 8-9% of said layer of
Component B, and from 3-20% of said layer of Component C, each
percentage based on the total weight or thickness of the plastic
film.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a heat-sealable plastic film having a
heat-sealable coating of a thermoplastic polystyrene resin.
2. Description of the Background
Plastic containers for the packaging of foodstuffs must in many
cases be sealed in an air-tight manner in order to ensure their
shelf lives. The seals used in many cases, in particular in the
packaging of dairy products, are aluminum foils coated with a
heat-sealable composition, which ensures adequate adhesion to a
wide variety of plastic containers. Since aluminum is insensitive
to heat, the heat-sealing operation can be carried out at any
desired temperature. It need only be ensured that the seal is of
sufficient quality that, on the one hand, the container is
sufficiently tightly sealed, ie., for example, does not burst when
dropped, but, on the other hand, can still easily be opened by hand
without the foil tearing. It is important here that the seal
satisfies this strength requirement over a certain processing
range, as required by commercially available sealing equipment.
In addition to aluminum foils, plastic films with a heat-sealable
coating are also used. The particular heat-sealable coating depends
on the type of plastic from which the container to be sealed is
produced. Thus, an acrylate resin-based coating with additives for
regulating the adhesive strength and the melting point is applied,
for example, to the hard PVC heat-sealable films most commonly used
for this purpose.
In contrast to aluminum foils, heat-sealable plastic films can also
be subjected to thermoforming. Shaped lids produced from plastic
films can thus also be heat-sealed to the container to be sealed,
but are suitable for re-sealing the package after removal of some
of the contents.
However, the PVC heat-sealable films currently used have some
disadvantages; they are relatively expensive, since they must be
provided with a heat-sealable coating, and the residual solvent
originating from the coating can only be removed from the plastic
using considerable effort. In addition, environmental
considerations increasingly mean that PVC is being replaced by
other thermoplastics in disposable packages.
DE-A 3,531,036 describes plastic films which can be produced by
coextrusion and have a heat-sealable coating comprising an impact
resistant polystyrene, a block copolymer and a lubricant.
Films made from amorphous polyethylene terephthalate with a
heat-sealable coating have also been disclosed. All these
heat-sealable plastic films must be provided with a thin
heat-stable protective coating on the side facing the sealer unit
in order to prevent adhesion of the plastic film to the sealer
jaws.
It has become apparent that a significantly narrower processing
latitude is available for use of all heat-sealable plastic films
compared with aluminum foils; if the sealing temperature and time
are inadequate, the plastic film does not permit sufficient heat
transmission, and adhesion of the film does not take place.
However, if the sealing temperature and time are excessive, the
plastic film melts and runs away under the pressure necessary for
sealing. It then becomes so thin in the sealing area that it tears
when only a small force is applied, and satisfactory opening and
re-sealing of the package is no longer possible. Between these two
extremes, there is, for all these heat-sealable plastic films,
usually only a narrow range of from 10.degree. to 20.degree. C.
which must be reliably observed in order to ensure trouble-free
production and satisfactory use of the sealed package. Practical
experience has shown that this is possible in a number of cases;
however, particularly in filling plants in which several pots are
sealed simultaneously, sufficiently reliable temperature control is
not always ensured since temperature differences of 20.degree. C.
and more are no exception between the various cavities.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to develop a
heat-sealable plastic film which contains, as heat-sealable
coating, a molding composition corresponding to DE-A 3,531,036, ie.
which can be produced by coextrusion and whose heat-sealable
coating is thus free from solvents, but in which the processing
latitude and thus the processing reliability are significantly
increased by other measures. In addition, films of this type
should, for example, satisfy the requirement that they facilitate
satisfactory production even in filling plants where pots filled
simultaneously in several cavities are sealed. Since the processing
temperature range cannot be extended downward due to the low
thermal conductivity of plastic films, this means that suitable
measures must be taken to ensure that satisfactory sealing and also
problem-free behavior of the sealed package in use are ensured,
even at relatively high sealing temperatures.
A further disadvantage of the currently used heat-sealable plastic
films mentioned is the necessity of preventing adhesion of the
plastic film to the sealer jaws by applying a protective coating.
This means that the film must be passed through a coater, which
represents a significant cost factor. It was thus a further object
of the present invention to produce a heat-sealable film by
coextrusion without any further subsequent coating steps, ie. it is
merely necessary to print the finished heat-sealable lids if
required.
We have found that these objects are achieved by the heat-sealable
plastic films produced in accordance with DE-A 3,531,036, which
satisfy these requirements in an excellent manner if they are
provided with a thin plastic coating having a high melting
point.
Accordingly, the invention directly provides a heat-sealable
plastic film produced by coextrusion or lamination, and preferably
containing 3 layers A, B and C, but at least 2 layers A and C, and,
if desired, at least one further layer of an adhesion promoter D,
which is intended to ensure the bonding between each two of the
layers A, B and C, where the sum of A, B (if present) and C can be
related to the thickness or weight and is in each case 100, and
which comprises from 1 to 50% of a layer of a heat-sealable, impact
resistant polystyrene A, up to 95% of a base layer B and from 1 to
99% of a high-melting plastic layer C.
The high-melting layer C can be applied, as stated, by lamination
or by coextrusion in one operation together with the production of
the heat-sealable plastic film. This avoids applying a protective
coating in a subsequent operation, and it has been shown,
unexpectedly, that a plastic layer of this type also prevents the
low-melting main layer of the heat-sealable plastic film running
off even at very high sealing temperatures, in contrast to the
current process, in which a protective coating is applied.
The result of this is that these heat-sealable plastic films
according to the invention can be used in virtually the same way
and in the same filling plants as heat-sealable aluminum foils
without tedious modifications to the filling plants being
necessary. The significant barriers to the range of applications of
heat-sealable plastic films of this type in industry are thus
removed. The advantages of the plastic films produced by this
process over those used hitherto are thus the problem-free
processing and use and the avoidance of subsequent applications of
a heat-sealing coating and the avoidance of hard-to-remove
solvents. The advantages over heat-sealable aluminum foils are the
much lower energy consumption for production, the possibility of
re-sealing used packages by means of thermoformed, heat-sealable
plastic lids, the better print quality and the avoidance of
solvents and their attendant removal.
However, it may in some cases be necessary to protect the print
with a protective coating in the area of the seal. It may also be
necessary, particularly in the case of additional plastic layers
having a melting point of below 250.degree. C., to further improve
the processing latitude by means of a coating of this type.
The structure of the plastic film according to the invention
involves components A, if desired B, and C. The film preferably
comprises 3 components in the following proportions, based on the
thickness or weight of the plastic film comprising A, B and C:
Component A: from 1 to 50%, preferably from 3 to 30% and in
particular from 3 to 15%
Component B: up to 95%, preferably from 50 to 90% and in particular
from 60 to 90%
Component C: from 1 to 99%, preferably from 1 to 30% and in
particular from 5 to 15%
The plastic film may also contain adhesion promoters D, which
ensure strong bonding of layers A, B and C to one another; their
amount is only small compared with A, B and C.
Materials which are known per se and are generally also
commercially available can be used for each of components A, B (if
present), C and the adhesion promoters D (if present) of the
plastic film according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Component A
Component A in the plastic film according to the invention has the
particular property of being heat-sealable. It is therefore used in
the broadest sense in the production of composites having a base,
in particular, a base film.
Suitable materials for component A in the plastic film according to
the invention are in particular molding compositions as described
in DE-A-3,531,036. Apart from additives such as stabilizers, these
molding compositions comprise 3 constituents; the essential
constituent (component A1) is impact-resistant polystyrene (for the
purposes of the invention, this is two-phase polystyrene comprising
a resin matrix and a soft phase).
The resin matrix of component A1 comprises a styrene polymer and
makes up from 60 to 95% by weight, preferably 80 to 95% by weight,
based on the component A. A suitable monomer for the resin matrix
is, in particular, styrene. However, it is also possible to use
.alpha.-methylstyrene or p-methylstyrene or mixtures of substituted
styrenes, but the exclusive use of styrene is preferred, and the
resin matrix thus preferably comprises polystyrene.
The resin matrix is produced in a manner known per se during the
production of component A by polymerizing, thermally or by means of
free radicals, the soft phase, ie. a rubber, for example, a
polydiene or a linear two-block copolymer made from diene or
mixtures together with the styrene monomers later making up the
resin matrix. During this, graft copolymers of the rubber (soft
phase) and ungrafted styrene polymers, the resin matrix, form.
The resin matrix can have a viscosity number .eta..sub.sp /c in the
range from 50 to 140, in particular in the range from 70 to 120.
This corresponds to mean molecular weights (M.sub.w) in the range
from 100,000 to 350,000, in particular from 150,000 to 300,000.
In the end, the soft phase is finely dispersed in the resin matrix.
The way in which a soft phase can be dispersed in a resin matrix is
known. The soft phase is present in the resin matrix in a
proportion of from 5 to 40% by weight, preferably from 5 to 20% by
weight, and has a mean particle size in the range from 0.01 to 10
.mu.m, preferably in the range from 0.3 to 8 .mu.m. In the particle
size range mentioned, the mean particle size, determined by
counting an electron photomicrograph, is thus a number average.
The soft phase is thus a graft copolymer comprising the monomer(s)
of the resin matrix, ie. in particular styrene, on rubber or a
mixture of rubbers, for example a mixture of an elastomeric, linear
styrene-butadiene two-block copolymer and polybutadiene.
The preferred soft phase is polybutadiene and the graft copolymer
thereof with styrene. Polybutadienes of the medium- or high-cis
type having molecular weights in the range from 70,000 to 450,000
(weight average) are particularly suitable.
Medium-cis-polybutadienes having molecular weights of from 300,000
to 400,000 are preferred.
Another highly suitable soft phase is a linear two-block copolymer
or graft copolymer thereof with styrene.
Elastomeric, linear two-block copolymers are obtained by anionic
polymerization with the aid of lithium initiators. Suitable
vinyl-aromatic monomers for the first block are styrene and
substituted styrenes. Specific examples are styrene, which is
preferably used alone, and o-, m- and p-methylstyrene. The second
block of the block copolymer preferably comprises only
butadiene.
The two-block copolymer can have, for example, a block polystyrene
content of from 40 to 90% by weight, based on the block copolymer
(remainder in each case butadiene). The block copolymer mentioned
has a so-called sharp transition. The preparation of block
copolymers of this type is known, for example, from A. Echte,
Angew. Makr. Chemie 58/59 (1977) 175. The block copolymer should
have a mean molecular weight (M.sub.w) of from 100,000 to 300,000,
preferably of from 150,000 to 250,000.
If mixtures of polydienes and linear block copolymers are used as
the rubber, the total polydiene content is calculated from the
proportion, for example, of the polybutadiene and, for example,
from the butadiene content of the two-block copolymer. This total
polydiene content should be in the range from 2 to 30% by weight,
preferably from 4 to 15% by weight, based on component A.
The following may serve as a practical example: a mixture of 8% by
weight of styrene-butadiene two-block copolymer having a butadiene
content of 50% by weight and 6% by weight of homopolybutadiene is
used. The total polybutadiene content is then 8.times.0.5=4% by
weight of polybutadiene from the two-block copolymer and 6% by
weight from the butadiene homopolymer, so the total polybutadiene
content is 10% by weight.
Impact resistant polystyrene resins prepared by the process
described in DE-A-1,770,392 or those having particularly good
stress cracking resistance (cf. DE-A-2,525,019) are particularly
preferred. It is also possible to use impact resistant polystyrene
having translucent properties, as described, for example, in
DE-A-2,613,352.
However, it is of course also possible to use styrene-butadiene
graft copolymers having a lower butadiene content, for example of
less than 40% by weight, or finally pure styrene homopolymer.
Usable heat-sealable compositions can then also be obtained through
an increased proportion of styrene-butadiene block copolymer.
However, this route is less economic.
The second constituent (independent of the abovementioned soft
phase) present in the specific component A of DE-A-3,531,036 is a
block copolymer A2 comprising styrene and butadiene. This comprises
at least one block of a vinyl-aromatic monomer, in particular from
the group comprising styrene, .alpha.-methylstyrene, ring-alkylated
styrene, such as p-methylstyrene, or mixtures thereof, and at least
one block comprising butadiene or isoprene or a mixture
thereof.
The block copolymer may comprise 2 or 3 blocks, and may be linear
or branched. Examples of suitable products and processes for its
preparation are described in: DE-A-1,084,919, DE-A-1,645,406,
DE-A-1,420,698 and U.S. Pat. No. 3,030,364. Branched products are
disclosed in DE-A-1,934,348, DE-A-1,959,922 and DE-A-2,550,227.
Processes for the preparation of branched block copolymers are also
described in the publications mentioned and in DE-A-3,248,746.
Styrene-isoprene block copolymers having a sharp or indistinct
transition are preferred.
The styrene-isoprene block copolymer here may be any block
copolymer, with no limitation on its structure, which is obtained
by (so-called anionic) solution polymerization using an alkyl
lithium compound as catalyst. Both elastomeric styrene-butadiene
and styrene-isoprene block copolymers and the resinous
styrene-butadiene and styrene-isoprene block copolymers having a
diene content of 50% by weight can be used. If elastomeric
styrene-diene block copolymers having a diene content of greater
than 50% by weight are used, their proportion in the mixture is
preferably from 20 to 40% by weight, but may be higher or lower, as
required.
Resinous styrene-diene block copolymers A2 having styrene contents
of greater than 50% by weight, in particular from 65 to 95% by
weight, should preferably be present in component A according to
the invention in an amount of from 40 to 60% by weight.
The third constituent of the specific component A is a lubricant
(component A3). Suitable lubricants are in principle all
low-molecular-weight substances which are compatible, in the amount
required, with components A1 and A2 in the molding composition.
These include, for example, mineral oils, aromatic or aliphatic
alcohols or esters, such as dodecyl alcohol, butyl stearate,
diethyl hexaphthalate, etc.
Further specific examples are ethylene oxide/propylene oxide block
copolymers; microhard waxes; ethylene-bisstearylamide (Acrawax);
metal soaps, in particular of the alkaline earth metals and of
zinc; mineral oils based on, in particular, naphthenic and
paraffinic hydrocarbons (in particular the technical grade and
medical white oils); and silicone oils having viscosities in the
range of from 0.5 to 50,000 mPas.
Further constituents of component A are customary additives, for
example release agents, antistatics, antioxidants, pigments or
fillers.
Component B
Component B, used if required, of the heat-sealable plastic film
according to the invention should be regarded as a base or base
film. Suitable base films are in principle all thermoplastics based
on styrene and butadiene, ie. impact resistant polystyrene,
styrene-butadiene block copolymers, and mixtures of styrene
homopolymers or styrene-butadiene graft copolymers with
styrene-butadiene block polymers. The heat-sealable layer adheres
to these substances without an adhesive. Examples of other suitable
thermoplastics are copolymers of styrene with acrylonitrile or
acrylonitrile and butadiene, and olefin polymers or polymers of
esters or amides. In these cases, however, it may be necessary to
bond the heat-sealable layer to the base film using an appropriate
adhesive or adhesion promoter.
Component B of the heat-sealable plastic film according to the
invention is preferably a high impact strength polystyrene resin
comprising a resin matrix and an elastomeric soft phase.
In this case, the resin matrix comprises the polymer of a
monovinyl-aromatic monomer, and makes up from 60 to 95% by weight,
preferably from 70 to 90% by weight, based on component B. The
monovinyl-aromatic monomer used is in particular styrene, but
substituted styrenes or copolymers of styrene with other suitable
monomers, such as maleic anhydride or methyl methacrylate, can also
be used as the resin matrix.
The corresponding soft phase of component B finely dispersed in the
resin matrix in the conventional manner and makes up from 5 to 40%
by weight, preferably from 8 to 20% by weight, based on B, of the
resin matrix. It has, for example, a mean particle size in the
range from 0.3 to 10 .mu.m.
The soft phase is a graft copolymer of the monomer(s) of the resin
matrix, ie. in particular, of styrene, on a rubber or a mixture of
rubbers; the rubber may, for example, be a mixture of an
elastomeric, linear styrene-butadiene two-block copolymer of the
A-B type and polybutadiene.
An example of a preferred soft phase is polybutadiene or a graft
copolymer thereof, preferably with styrene. The graft base is
generally a polymer having from 4 to 5 carbon atoms; in particular,
polybutadiene of the medium- or high-cis type having a molecular
weight in the range from 70,000 to 450,000 (weight average) is
suitable. Medium-cis-polybutadiene having a molecular weight of
from 300,000 to 400,000 is preferred.
The soft phase used may also be a styrene-butadiene block
copolymer. This can be employed alone or in addition to the graft
copolymer. Examples are two-block copolymers A-B comprising a
styrene block A and a polybutadiene block B. The two-block
copolymer may, for example, have a block styrene content of from 30
to 90% by weight, and the remainder is butadiene.
If a mixture of polydiene and a linear block copolymer is used, the
polydiene represents the entire amount of, for example,
polybutadiene and, for example, the butadiene content in the
two-block copolymer. The total amount of polydiene should be in the
range from 4 to 40%, preferably from 4 to 20%, by weight, based on
component B.
Component B is particularly preferably impact resistant polystyrene
prepared by the process described in German Published Application
DE-AS 1,770,392.
Component C
Component C should have a high melting point and prevent adhesion
of the heat-sealable films to the sealer head. However, component C
of the plastic films replaces not only a protective coating, but
also increases, in a surprising manner, the processing latitude of
the heat-sealable plastic film to such an extent that the base
layer is prevented from running off, even at very high sealing
temperatures and very long sealing times, and interruption-free
processing and problem-free use of the heat-sealing films is thus
made possible for the first time. All thermoplastics whose melting
point or softening range is above 200.degree. C., preferably above
230.degree. C., are suitable as component C of the heat-sealable
plastic films according to the invention. Thus, for example, the
plastics listed in the table below are suitable as component C.
______________________________________ Plastic, conventional
Chemical Melting name composition point
______________________________________ Nylon 6 Polycaprolactam 220
Nylon 6,6 Polyhexamethylene- 255 adipamide Nylon copoly- Base
hexamethylene- 298 merized with diamine, caprolactam, butylene
tereph- terephthalic acid thalate Polybutylene Condensate made from
220 to 225 terephthalate terephthalic acid or dimethylphthalic acid
and 1,4-butanediol ______________________________________
The following may also be mentioned: polyphenylene oxide and
mixtures thereof with polystyrene, polysulfones, polyether
sulfones, polyether ketones, LC polymers, polyether imides,
crystalline polyethylene terephthalate, polyphenylene sulfide,
polyamide imide copolymers or polyamides having the structure of
Nylon 6,12 (polyhexamethylene dodecamide), 11
(poly-.omega.-aminoundecanoic acid), 4.6 (polytetramethylene
adipamide) etc., so long as the melting point or softening range is
above 200.degree. C.
Component D
Depending on the type of components B and C, an adhesive or
adhesion promoter, preferably thermoplastic, may be necessary to
provide a strong bond between the individual layers of the 3
components A, B and C. Experience has shown that adhesion promoters
are not necessary for bonding A to B if component B is a
styrene-butadiene polymer containing less than 15% of other
comonomers. For bonding component A to polyolefins, ethylene-vinyl
acetate copolymers having a vinyl acetate content of greater than
10 % or styrene-butadiene block copolymers predominantly comprising
butadiene have otherwise proven suitable. Component A can be bonded
to polyamide using styrene-maleic anhydride copolymers or to
polyesters, such as polyethylene terephthalate and polybutylene
terephthalate, using styrene-butadiene block copolymers.
Component D of the heat-sealable plastic film according to the
invention may also be, for example, a further layer of a
thermoplastic having high gas- and/or steam-impermeability
incorporated as a gas or steam barrier. This layer protects the
contents, for example, from drying out or from the effects of
oxygen. Examples are layers of ethylene-vinyl alcohol copolymers
which have high gas impermeability or layers of thermoplastic PVDC
copolymers having high gas and steam impermeability, etc. If
necessary, a layer of this type should likewise be bonded to the
adjacent components using a suitable adhesion promoter.
Component D may also be a substantially opaque or otherwise colored
layer which protects the contents, for example, against the effects
of light. To this end, suitable pigments, preferably carbon black
or mixtures of carbon black with titanium dioxide or further
pigments, are added to a material, for example of components B or
C, and an additional pigmented layer is introduced; this
facilitates adhesion without adhesion promoters.
Finally, it is possible to use a further component D comprising,
for example, layers having an anti-electrostatic finish, layers
having improved printability or improved scratch resistance, layers
having high impact resistance, etc.
The heat-sealable plastic film according to the invention is
preferably produced by coextrusion. This process is known.
Coextrusion can be carried out at bulk temperatures of from around
170.degree. C. to 350.degree. C. The individual components A-D are
melted in different extruders and either combined in multiple flat
film dies or annular dies or the layer(s) is(are) combined in an
adapter.
It is also possible to apply one or more layers onto the other
extruded layer(s) by lamination.
During extrusion, the thickness of the individual layers can be
matched to the requirements on the finished films, for example, by
varying the extruder speed.
Use of the Heat-Sealable Plastic Film
The heat-sealable films produced in this way are sealed onto
containers made of styrene polymers, for example styrene
homopolymers, styrene-butadiene graft polymers or block copolymers,
or mixtures of these substances, on conventional sealers used for
heat sealing. The containers may also comprise other styrene
polymers, for example those containing a certain proportion of
copolymers such as acrylonitrile. Multilayer films are very
frequently used for packages. These are also suitable for sealing
using these heat-sealable plastic films, with the proviso that the
inner layer onto which the lid is sealed comprises a styrene
polymer which can be heat-sealed against component A of the
heat-sealable film.
For a prespecified shape of the sealing head, the sealing
conditions are described by means of temperature of the sealing
head, the pressure during sealing and the sealing time. Due to the
low thermal conductivity of the heat-sealable plastic film compared
with aluminum foils, the sealing conditions are highly dependent on
the thickness of the heat-sealable plastic films. For example, a
satisfactory sealing can be achieved for a 0.1 mm thick film in
only 0.2 to 0.3 second at a sealing temperature of 200.degree. C.
and a sealing pressure of 2 bar, whereas a sealing time of 0.8 to 1
second is required under the same conditions for a 0.25 mm thick
film.
Depending on the type of component C of the heat-sealable plastic
film, suitable sealing conditions are a pressure of from 1 to 5
bar, a sealing temperature of from 140.degree. C. to 280.degree. C.
and a sealing time of from 0.1 to 2 seconds.
EXAMPLE 1
A heat-sealable plastic film is produced from the following
components by coextrusion.
Component A: heat-sealable layer corresponding to the mixture used
in the examples of German Laid-Open Application DE-OS
3,531,036.
Component B: high impact strength polystyrene containing 8% of
butadiene (Vicat softening point 95.degree. C. (DIN 53 460, method
A), melt flow index 3 g/10 min in accordance with DIN 53 735,
200.degree. C./5 kg) blended with 15% of a styrene-butadiene
two-block copolymer containing 72% of butadiene.
Component C: nylon 6.6, melting point 260.degree. C., determined in
accordance with ISO 1218, method A.
Component D: copolymer comprising 80% of ethylene, 10% of vinyl
acetate and 10% of maleic anhydride, for use as an adhesion
promoter between layers B and C.
Component B is fed to a main extruder, and components A, C and D
are fed to a multiple flat film die, each through an ancillary
extruder. The processing temperatures for the molding compositions
were:
Component A: 189.degree. C.; component B: 210.degree. C.; component
C: 280.degree. C.;
Component D: 240.degree. C.; and the layer structure of the film
was as follows:
Component A: 0.01 mm; component B: 0.23 mm; component C: 0.02
mm;
Component D: 0.0 1 mm.
EXAMPLES 2 TO 12
Examples 2 to 12 were produced in the same equipment and can be
summarized in tabular form (Table 1). Component A was the material
of Example 1.
______________________________________ Ex- am- Component Component
ple A Component B Component C D
______________________________________ 2 0.01 mm 0.02 mm high 0.01
mm co- 0.03 mm impact polymer com- Nylon 6,6 strength prising 80%
polystyrene of ethylene (as in Ex. 1) and 20% of vinyl acetate 3
0.015 mm 0.23 mm high 0.013 mm co- 0.03 mm impact polymer com-
Nylon 6,6 strength prising 80% polystyrene ethylene, (corresponding
10% vinyl to Ex. 1) acetate and 10% of maleic anhydride 4 0.01 mm
0.08 mm high As for Ex. 3 0.015 mm strength Nylon 6,6 polystyrene
(corresponding to Ex. 1) 5 0.01 mm 0.08 mm high As for Ex. 3 0.01
mm impact Nylon co- strength polymer, polystyrene melting point
(corresponding 298.degree. C. to Ex. 1) 6 0.01 mm 0.2 mm high 0.012
mm 0.02 mm impact styrene- polybutylene strength butadiene
terephthalate, polystyrene block co- m.p. 225.degree. C.
(correspond- polymer con- to Ex. 1) taining 30% of butadiene 7 0.01
mm 0.08 mm high As for Ex. 6 As for Ex. 6 impact strength
polystyrene (corresponding to Ex. 1) 8 0.01 mm 0.25 mm high As for
Ex. 3 0.02 mm impact Nylon strength copolymer, polystyrene m.p.
298.degree. C. 9 0.01 mm As for Ex. 8 As for Ex. 8 0.03 mm Nylon 6
10 0.01 mm No layer B As for Ex. 8 0.06 mm Nylon 6,6 11 0.01 mm No
layer B As for Ex. 3 0.04 mm nylon copolymer, m.p. 298.degree. C.
12 0.01 mm No layer B As for Ex. 6 0.06 mm polybutylene
terephthalate Comparison experiments 13 0.02 mm 0.24 mm Standard
commercial as for Ex. 1 overcoating with protective coating 14
0.015 mm 0.11 mm Standard commercial as for Ex. 1 overcoating with
protective coating 15 0.04 mm thick aluminum foil with
acrylate-based heat- sealable coating 16 0.1 mm thick PVC film with
acrylate-based heat-sealable coating and commercially available
protective coating 17 0.23 mm thick PVC film with acrylate-based
heat-sealable coating and commercially available protective coating
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Heat-sealing experiments were carried out using the heat-sealable
plastic films produced as described in Examples 1 to 12. Some of
the heat-sealable films produced as described in Examples 1 to 12
were additionally coated with a heat-resistant coating (in each
case variant a), and in comparison experiments 13 and 14,
heat-sealable plastic films corresponding to DE-A-3,531,036, ie.
without components C and D according to the invention, but with a
heat-resistant coating, were used. In addition, commercially
available aluminum heat-sealable foils (Example 15) and
commercially available PVC heat-sealable films (Examples 16 and 17)
were included in the comparison experiments. Each of these was also
coated with a protective coating and with an acrylate-based
heat-sealable coating.
The parameters given in the examples and comparison experiments
were determined as follows:
1. Sealing conditions with the sealing head adjusted differently
with respect to temperature (.degree.C.), pressure (bar) and time
(sec). The films were heat-sealed onto commercially available
thermoformed packaging pots made from impact resistant polystyrene,
butadiene content 6%, Vicat softening point 90.degree. C. (DIN 53
460 method A), melt flow index 4 g/10 min (MFI: 200.degree. C./5 in
accordance with DIN 53 735).
2. Determination of the bursting pressure
Packaging pots were sealed under various sealing conditions. In
order to determine the strength of the seal bond, a bursting
pressure test was carried out. To this end, a measurable excess
pressure was produced in the sealed container by means of
compressed air using suitable equipment. The apparatus essentially
comprises a manometer with trailing pointer and a compressed air
line connected to an aperture in the pot by means of a rubber seal.
The pressure was increased until the seal weld opened. The maximum
pressure achieved is referred to as the bursting pressure.
3. Determination of the drop height
Packaging pots of standardized dimensions were filled with water
and heat-sealed under various conditions. The maximum drop height
in cm before the seal bursts was determined.
4. Determination of the peel strength from the pot Packaging pots
were heat-sealed under various conditions, and the peel strength of
the heat-sealable film from the pot was determined using a
specially designed apparatus. To this end, the pots were clamped
onto the stage of a universal pressure and tensile testing machine.
A projecting corner of the heat-sealed lid was held and connected
to the upper clamp jaw of the testing machine via an extension.
The force was built up at a load rate of 10 mm/mm and the lid
peeled off. The force in [N] before the first tear and the tear
propagation force on the seal weld in the center of the lid were
determined.
TABLE 2
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Peel strength Tear propa- Residual thickness gation force Sealing
conditions of the heat-sealable Bursting Drop Force before in the
center Temperature Pressure Time film at the seal edge pressure
height 1st tear of the lid Example [.degree.C.] [bar] [sec] [% of
initial value] [bar] [cm] [N] [N]
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1 200 2 1 85 1.55 110 18.2 2.2 1 220 2 1 84 1.61 108 20.5 2.4 1 240
2 0.8 80 1.67 115 22.3 2.5 1a 250 2 0.8 78 1.51 96 24.5 2.7 2 200 2
1 87 1.52 105 17.3 1.9 2 220 2 1 86 1.57 115 21.4 2.3 2 240 2 0.8
82 1.68 109 21.8 2.5 2a 250 2 0.8 80 1.70 112 23.6 2.8 3 200 2 1 80
1.71 104 19.1 1.9 3 220 2 1 45 1.42 106 16.2 1.8 3a 240 2 0.8 10
1.1 40 4.8 <0.5 4 200 2 0.3 87 1.72 103 19.5 2.4 4 220 2 0.3 85
1.74 105 17.6 2.7 4 240 2 0.3 81 1.58 107 21.3 3.1 4a 250 2 0.3 79
1.68 102 24.4 3.3 5 200 2 0.3 88 1.68 98 17.9 2.1 5 240 2 0.3 88
1.69 101 23.3 2.4 5 260 2 0.2 85 1.75 102 19.4 2.8 5 280 2 0.2 80
1.70 98 21.3 3.1 5a 290 2 0.2 75 1.61 105 24.3 3.5 6 200 2 1 85
1.61 100 21.3 2.2 6 220 2 1 80 1.68 89 20.0 2.3 6a 240 2 0.8 15
1.09 15 3.2 <0.5 7 200 2 0.3 88 1.72 98 22.9 1.8 7 220 2 0.3 80
1.76 110 24.7 2.1 7a 240 2 0.3 12 1.21 24 7.8 <0.5 8 200 2 1 88
1.68 105 20.1 2.1 8 240 2 0.8 87 1.72 107 21.1 2.3 8 260 2 0.6 84
1.81 109 23.3 2.4 8 280 2 0.6 77 1.82 111 26.8 2.7 8a 290 2 0.5 75
1.82 103 25.7 3.6 9 200 2 1 82 1.68 107 22.6 2.9 9 220 2 1 79 1.65
104 21.8 3.2 9a 240 2 0.8 18 1.17 21 4.2 0.5 10 200 2 0.2 91 1.63
102 23.3 2.2 10 220 2 0.2 89 1.68 100 22.3 2.5 10 240 2 0.2 88 1.72
112 24.1 3.0 10a 250 2 0.2 86 1.85 116 27.2 3.3 11 200 2 0.2 93
1.70 103 24.1 2.5 11 240 2 0.2 93 1.75 98 25.2 2.7 11 260 2 0.2 91
1.73 108 25.5 3.5 11 280 2 0.2 90 1.81 109 28.1 3.9 11a 290 2 0.2
85 1.91 116 30.1 4.5 12 200 2 0.3 90 1.65 100 22.7 2.5 12 220 2 0.3
85 1.70 97 23.5 2.9 12a 240 2 0.3 10 <1.1 <10 2.4 <0.5 13
200 2 1 72 1.71 105 24.3 2.1 13 220 2 1 45 1.75 110 24.2 2.5 13 240
2 0.8 12 <1.1 <10 3.8 <0.5 14 200 2 0.3 68 1.70 103 20.2
2.4 14 220 2 0.3 42 1.60 98 18.3 2.4 14 240 2 0.3 14 <1.1 <10
2.7 <0.5 15 220 2 0.2 98 1.51 87 27.2 3.2 15 240 2 0.2 98 1.48
95 26.3 3.2 15 260 2 0.2 98 1.55 103 29.5 3.5 15 280 2 0.2 98 1.60
105 30.0 3.7 16 200 2 0.3 91 1.43 75 17.2 0.8 16 220 2 0.3 78 1.45
80 16.5 0.9 16 240 2 0.3 46 1.28 29 9.7 0.8 17 200 2 1.0 92 1.39 74
12.5 0.6 17 220 2 1.0 75 1.42 70 16.3 0.9 17 240 2 0.8 45 1.21 32
10.1 0.8 18 200 2 1.0 85 1.28 58 12.4 0.5 18 220 2 1.0 65 1.31 64
16.4 0.7 18 240 2 0.8 32 1.30 20 <10 0.7
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